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<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> Attributes</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Types</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typedef.html" title="cpp/language/typedef"> <code>typedef</code> declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> Type alias declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="declarations.html" title="cpp/language/declarations"> Declarations</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Overview </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> declaration syntax</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html#Specifiers" title="cpp/language/declarations"> decl-specifier-seq</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html#Declarators" title="cpp/language/declarations"> declarator</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Specifiers </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="typedef.html" title="cpp/language/typedef"><span class="t-lines"><span>typedef</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="inline.html" title="cpp/language/inline"><span class="t-lines"><span>inline</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> <code>virtual</code> function specifier</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"> <code>explicit</code> function specifier</a>   </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="friend.html" title="cpp/language/friend"><span class="t-lines"><span>friend</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="constexpr.html" title="cpp/language/constexpr"><span class="t-lines"><span>constexpr</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="consteval.html" title="cpp/language/consteval"><span class="t-lines"><span>consteval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="constinit.html" title="cpp/language/constinit"><span class="t-lines"><span>constinit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> storage class specifiers</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="tu_local.html" title="cpp/language/tu local"> translation-unit-local</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="class.html" title="cpp/language/class"><span class="t-lines"><span>class/struct</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="union.html" title="cpp/language/union"><span class="t-lines"><span>union</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="enum_class.html" title="cpp/language/enum"><span class="t-lines"><span>enum</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="decltype.html" title="cpp/language/decltype"><span class="t-lines"><span>decltype</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="auto.html" title="cpp/language/auto"><span class="t-lines"><span>auto</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="alignas.html" title="cpp/language/alignas"><span class="t-lines"><span>alignas</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="cv.html" title="cpp/language/cv"><span class="t-lines"><span>const</span><span>volatile</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier"> elaborated type specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> attributes </a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Declarators </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="reference.html" title="cpp/language/reference"> reference </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="pointer.html" title="cpp/language/pointer"> pointer </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="array.html" title="cpp/language/array"> array </a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Block declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> simple-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> →<a href="structured_binding.html" title="cpp/language/structured binding"> structured binding declaration</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> <code>alias</code> declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace_alias.html" title="cpp/language/namespace alias"> namespace alias definition </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"> using-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html#Using-directives" title="cpp/language/namespace"> using-directive</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_assert.html" title="cpp/language/static assert"> <code>static_assert</code> declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> asm-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="enum_class.html" title="cpp/language/enum"> opaque enum declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Other declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html" title="cpp/language/namespace"> namespace definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> class template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> function template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template"> explicit template instantiation</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> explicit template specialization</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="language_linkage.html" title="cpp/language/language linkage"> linkage specification</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> empty declaration</a> </td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>The storage class specifiers are a part of the <span class="t-spar">decl-specifier-seq</span> of a name's <a href="declarations.html" title="cpp/language/declarations">declaration syntax</a>. Together with the <a href="scope.html" title="cpp/language/scope">scope</a> of the name, they control two independent properties of the name: its <i>storage duration</i> and its <i>linkage</i>. 
</p>
 <table class="t-rev-begin">
<dl><dd><ul><li><span class="t-rev-inl t-until-cxx11"><span><code><b>auto</b></code> or </span> <span><span class="t-mark-rev t-until-cxx11">(until C++11)</span></span></span>no specifier - <i>automatic</i> storage duration.
</li></ul>
</dd></dl>
<tr class="t-rev t-until-cxx17"><td>
<dl><dd><ul><li><code><b>register</b></code> - <i>automatic</i> storage duration. Also hints to the compiler to place the object in the processor's register. <span class="t-mark">(deprecated)</span>
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
</table>
<dl><dd><ul><li><code><b>static</b></code> - <i>static</i> or <i>thread</i> storage duration and <i>internal</i> linkage (or <i>external</i> linkage for static class members not in an anonymous namespace).
</li><li><code><b>extern</b></code> - <i>static</i> or <i>thread</i> storage duration and <i>external</i> linkage.
</li></ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<dl><dd><ul><li><code><b>thread_local</b></code> - <i>thread</i> storage duration.
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<dl><dd><ul><li><code><b>mutable</b></code> - does not affect storage duration or linkage. See <a href="cv.html" title="cpp/language/cv">const/volatile</a> for the explanation.
</li></ul>
</dd></dl>
<p><br>
Only one storage class specifier may appear in a declaration <span class="t-rev-inl t-since-cxx11"><span>except that <code>thread_local</code> may be combined with <code>static</code> or with <code>extern</code></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.
</p>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11"><td>
<div class="t-li1"><span class="t-li">1)</span> The <code>auto</code> specifier was only allowed for objects declared at block scope or in function parameter lists. It indicated automatic storage duration, which is the default for these kinds of declarations. The meaning of this keyword was changed in C++11.
</div></td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
<tr class="t-rev t-until-cxx17"><td>
<div class="t-li1"><span class="t-li">2)</span> The <code>register</code> specifier is only allowed for objects declared at block scope and in function parameter lists. It indicates automatic storage duration, which is the default for these kinds of declarations. Additionally, the presence of this keyword may be used as a hint for the optimizer to store the value of this variable in a CPU register. This keyword is deprecated.
</div></td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
</table>
<div class="t-li1"><span class="t-li">3)</span> The <code>static</code> specifier is only allowed in the declarations of objects (except in function parameter lists), declarations of functions (except at block scope), and declarations of anonymous unions. When used in a declaration of a class member, it declares a <a href="static.html" title="cpp/language/static">static member</a>. When used in a declaration of an object, it specifies static storage duration (except if accompanied by <code>thread_local</code>). When used in a declaration at namespace scope, it specifies internal linkage.</div>
<div class="t-li1"><span class="t-li">4)</span> The <code>extern</code> specifier is only allowed in the declarations of variables and functions (except class members or function parameters). It specifies external linkage, and does not technically affect storage duration, but it cannot be used in a definition of an automatic storage duration object, so all <code>extern</code> objects have static or thread durations. In addition, a variable declaration that uses <code>extern</code> and has no initializer is not a <a href="definition.html" title="cpp/language/definition">definition</a>.</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<div class="t-li1"><span class="t-li">5)</span> The <code>thread_local</code> keyword is only allowed for objects declared at namespace scope, objects declared at block scope, and static data members. It indicates that the object has thread storage duration. If <code>thread_local</code> is the only storage class specifier applied to a block scope variable, <code>static</code> is also implied. It can be combined with <code>static</code> or <code>extern</code> to specify internal or external linkage (except for static data members which always have external linkage) respectively.</div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h4><span class="mw-headline" id="Storage_duration">Storage duration</span></h4>
<p>All <a href="objects.html" title="cpp/language/object">objects</a> in a program have one of the following storage durations:
</p>
<dl><dd><ul><li><i><b>automatic</b></i> storage duration. The storage for the object is allocated at the beginning of the enclosing code block and deallocated at the end. All local objects have this storage duration, except those declared <code>static</code>, <code>extern</code> or <code>thread_local</code>.
</li></ul>
</dd></dl>
<dl><dd><ul><li><i><b>static</b></i> storage duration. The storage for the object is allocated when the program begins and deallocated when the program ends. Only one instance of the object exists. All objects declared at namespace scope (including global namespace) have this storage duration, plus those declared with <code>static</code> or <code>extern</code>. See <a href="initialization.html#Non-local_variables" title="cpp/language/initialization">Non-local variables</a> and <a href="storage_duration.html#Static_local_variables">Static local variables</a> for details on initialization of objects with this storage duration.
</li></ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<dl><dd><ul><li><i><b>thread</b></i> storage duration. The storage for the object is allocated when the thread begins and deallocated when the thread ends. Each thread has its own instance of the object. Only objects declared <code>thread_local</code> have this storage duration. <code>thread_local</code> can appear together with <code>static</code> or <code>extern</code> to adjust linkage. See <a href="initialization.html#Non-local_variables" title="cpp/language/initialization">Non-local variables</a> and <a href="storage_duration.html#Static_local_variables">Static local variables</a> for details on initialization of objects with this storage duration.
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<dl><dd><ul><li><i><b>dynamic</b></i> storage duration. The storage for the object is allocated and deallocated upon request by using <a href="../memory.html" title="cpp/memory">dynamic memory allocation</a> functions. See <a href="new.html" title="cpp/language/new">new-expression</a> for details on initialization of objects with this storage duration.
</li></ul>
</dd></dl>
<p>The storage duration of <a href="objects.html#Subobjects" title="cpp/language/object">subobjects</a> and reference members is that of their complete object.
</p>
<h4><span class="mw-headline" id="Linkage">Linkage</span></h4>
<p>A name that denotes object, reference, function, type, template, namespace, or value, may have <i>linkage</i>. If a name has linkage, it refers to the same entity as the same name introduced by a declaration in another scope. If a variable, function, or another entity with the same name is declared in several scopes, but does not have sufficient linkage, then several instances of the entity are generated.
</p><p>The following linkages are recognized:
</p>
<h5><span class="mw-headline" id="no_linkage">no linkage</span></h5>
<p>The name can be referred to only from the scope it is in.
</p><p>Any of the following names declared at block scope have no linkage:
</p>
<ul><li> variables that aren't explicitly declared <code>extern</code> (regardless of the <code>static</code> modifier);
</li><li> <a href="class.html#Local_classes" title="cpp/language/class">local classes</a> and their member functions;
</li><li> other names declared at block scope such as typedefs, enumerations, and enumerators.
</li></ul>
<p>Names not specified with external<span class="t-rev-inl t-since-cxx20"><span>, module,</span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span> or internal linkage also have no linkage, regardless of which scope they are declared in.
</p>
<h5><span class="mw-headline" id="internal_linkage">internal linkage</span></h5>
<p>The name can be referred to from all scopes in the current translation unit.
</p><p>Any of the following names declared at namespace scope have internal linkage:
</p>
<ul><li> variables<span class="t-rev-inl t-since-cxx14"><span>, variable templates</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>, functions, or function templates declared <code>static</code>;
</li><li> non-volatile<span class="t-rev-inl t-since-cxx14"><span> non-template</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span><span class="t-rev-inl t-since-cxx17"><span> non-inline</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span><span class="t-rev-inl t-since-cxx20"><span> non-<a href="modules.html" title="cpp/language/modules">exported</a></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span> <a href="cv.html" title="cpp/language/cv">const-qualified</a> variables <span class="t-rev-inl t-since-cxx11"><span>(including <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a>)</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> that aren't declared <code>extern</code> and aren't previously declared to have external linkage;
</li><li> data members of <a href="union.html" title="cpp/language/union">anonymous unions</a>.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>In addition, all names declared in <a href="namespace.html" title="cpp/language/namespace">unnamed namespace</a> or a namespace within an unnamed namespace, even ones explicitly declared <code>extern</code>, have internal linkage.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h5><span class="mw-headline" id="external_linkage">external linkage</span></h5>
<p>The name can be referred to from the scopes in the other translation units. Variables and functions with external linkage also have <a href="language_linkage.html" title="cpp/language/language linkage">language linkage</a>, which makes it possible to link translation units written in different programming languages.
</p><p>Any of the following names declared at namespace scope have external linkage, unless they are declared in an unnamed namespace<span class="t-rev-inl t-since-cxx20"><span> or their declarations are attached to a named module and are not exported</span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>:
</p>
<ul><li> variables and functions not listed above (that is, functions not declared <code>static</code>, non-const variables not declared <code>static</code>, and any variables declared <code>extern</code>);
</li><li> enumerations;
</li><li> names of classes, their member functions, static data members (const or not), nested classes and enumerations, and functions first introduced with <a href="friend.html" title="cpp/language/friend">friend</a> declarations inside class bodies;
</li><li> names of all templates not listed above (that is, not function templates declared <code>static</code>).
</li></ul>
<p>Any of the following names first declared at block scope have external linkage:
</p>
<ul><li> names of variables declared <code>extern</code>;
</li><li> names of functions.
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<h5> <span class="mw-headline" id="module_linkage">module linkage</span></h5>
<p>The name can be referred to only from the scopes in the same module unit or in the other translation units of the same named module.
</p><p>Names declared at namespace scope have module linkage if their declarations are attached to a named module and are not exported, and don't have internal linkage.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: add the description of the behavior when an entity is declared with different linkages in the same translation unit (6.6 paragraph 6), note the difference between C++20 (ill-formed) and the current draft (well-formed) </td></tr></table>
<h3><span class="mw-headline" id="Static_local_variables">Static local variables</span></h3>
<p>Variables declared at block scope with the specifier <code>static</code> <span class="t-rev-inl t-since-cxx11"><span>or <code>thread_local</code></span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> have static <span class="t-rev-inl t-since-cxx11"><span>or thread</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span> storage duration but are initialized the first time control passes through their declaration (unless their initialization is <a href="zero_initialization.html" title="cpp/language/zero initialization">zero-</a> or <a href="constant_initialization.html" title="cpp/language/constant initialization">constant-initialization</a>, which can be performed before the block is first entered). On all further calls, the declaration is skipped.
</p><p>If the initialization <a href="throw.html" title="cpp/language/throw">throws an exception</a>, the variable is not considered to be initialized, and initialization will be attempted again the next time control passes through the declaration.
</p><p>If the initialization recursively enters the block in which the variable is being initialized, the behavior is undefined.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with <span class="t-lc"><a href="../thread/call_once.html" title="cpp/thread/call once">std::call_once</a></span>).
</p><p>Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>The destructor for a block-scope static variable <a href="../utility/program/exit.html" title="cpp/utility/program/exit">is called at program exit</a>, but only if the initialization took place successfully.
</p><p>Function-local static objects in all definitions of the same <a href="inline.html" title="cpp/language/inline">inline function</a> (which may be implicitly inline) all refer to the same object defined in one translation unit, as long as the function has external linkage.
</p>
<h3><span class="mw-headline" id="Translation-unit-local_entities">Translation-unit-local entities</span></h3>
<p>The concept of translation-unit-local entities is standardized in C++20, see <a href="tu_local.html" title="cpp/language/tu local">this page</a> for more details.
</p><p>An entity is <i>translation-unit-local</i> (or <i>TU-local</i> for short) if
</p>
<ul><li> it has a name with internal linkage, or
</li><li> it does not have a name with linkage and is introduced within the definition of a TU-local entity, or
</li><li> it is a template or template specialization whose template argument or template declaration uses a TU-local entity.
</li></ul>
<p>Bad things (usually violation of <a href="definition.html" title="cpp/language/definition">ODR</a>) can happen if the type of a non-TU-local entity depends on a TU-local entity, or if a declaration of<span class="t-rev-inl t-since-cxx17"><span>, or a <a href="ctad.html" title="cpp/language/ctad" class="mw-redirect">deduction guide</a> for,</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> a non-TU-local entity names a TU-local entity outside its
</p>
<ul><li> function-body for a non-inline function or function template
</li><li> initializer for a variable or variable template
</li><li> friend declarations in a class definition
</li><li> use of value of a variable, if the variable is <a href="constant_expression.html#Usable_in_constant_expressions" title="cpp/language/constant expression">usable in constant expressions</a>
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>Such uses are disallowed in a <a href="modules.html" title="cpp/language/modules">module interface unit</a> (outside its private-module-fragment, if any) or a module partition, and are deprecated in any other context.
</p><p>A declaration that appears in one translation unit cannot name a TU-local entity declared in another translation unit that is not a header unit. A declaration instantiated for a <a href="templates.html" title="cpp/language/templates">template</a> appears at the point of instantiation of the specialization.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>Names at the top-level namespace scope (file scope in C) that are <code>const</code> and not <code>extern</code> have external linkage in C, but internal linkage in C++.
</p><p>Since C++11, <code>auto</code> is no longer a storage class specifier; it is used to indicate type deduction. 
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17"><td>
<p>In C, the address of a <code>register</code> variable cannot be taken, but in C++, a variable declared <code>register</code> is semantically indistinguishable from a variable declared without any storage class specifiers.
</p>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<p>In C++, unlike C, variables cannot be declared <code>register</code>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<p>Names of <code>thread_local</code> variables with internal or external linkage referred from different scopes may refer to the same or to different instances depending on whether the code is executing in the same or in different threads.
</p><p>The <code>extern</code> keyword can also be used to specify <a href="language_linkage.html" title="cpp/language/language linkage">language linkage</a> and <a href="class_template.html" title="cpp/language/class template">explicit template instantiation declarations</a>, but it's not a storage class specifier in those cases (except when a declaration is directly contained in a language linkage specification, in which case the declaration is treated as if it contains the <code>extern</code> specifier).
</p><p><a href="cv.html" title="cpp/language/cv">The keyword <code>mutable</code></a> is a storage class specifier in the C++ language grammar, although it doesn't affect storage duration or linkage.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: the rules about re-declaring names in the same TU </td></tr></table>
<p>Storage class specifiers, except for <code>thread_local</code>, are not allowed on <a href="template_specialization.html" title="cpp/language/template specialization">explicit specializations</a> and <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template">explicit instantiations</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> S
<span class="br0">{</span>
    thread_local <span class="kw4">static</span> <span class="kw4">int</span> tlm<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
thread_local <span class="kw4">int</span> S<span class="sy1">&lt;</span><span class="kw4">float</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">tlm</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// "static" does not appear here</span></pre></div></div>
<h3><span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/auto.html" title="cpp/keyword/auto"><tt>auto</tt></a>,
<a href="../keyword/register.html" title="cpp/keyword/register"><tt>register</tt></a>,
<a href="../keywords/static.html" title="cpp/keyword/static"><tt>static</tt></a>,
<a href="../keyword/extern.html" title="cpp/keyword/extern"><tt>extern</tt></a>,
<a href="../keyword/thread_local.html" title="cpp/keyword/thread local"><tt>thread_local</tt></a>,
<a href="../keyword/mutable.html" title="cpp/keyword/mutable"><tt>mutable</tt></a>
</p>
<h3><span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;thread&gt;</span>
<span class="co2">#include &lt;mutex&gt;</span>
 
thread_local <span class="kw4">unsigned</span> <span class="kw4">int</span> rage <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> 
<a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a> cout_mutex<span class="sy4">;</span>
 
<span class="kw4">void</span> increase_rage<span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> thread_name<span class="br0">)</span>
<span class="br0">{</span>
    <span class="sy2">++</span>rage<span class="sy4">;</span> <span class="co1">// modifying outside a lock is okay; this is a thread-local variable</span>
    <a href="../thread/lock_guard.html"><span class="kw2170">std::<span class="me2">lock_guard</span></span></a><span class="sy1">&lt;</span><a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a><span class="sy1">&gt;</span> lock<span class="br0">(</span>cout_mutex<span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Rage counter for "</span> <span class="sy1">&lt;&lt;</span> thread_name <span class="sy1">&lt;&lt;</span> <span class="st0">": "</span> <span class="sy1">&lt;&lt;</span> rage <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../thread/thread.html"><span class="kw2150">std::<span class="me2">thread</span></span></a> a<span class="br0">(</span>increase_rage, <span class="st0">"a"</span><span class="br0">)</span>, b<span class="br0">(</span>increase_rage, <span class="st0">"b"</span><span class="br0">)</span><span class="sy4">;</span>
 
    <span class="br0">{</span>
        <a href="../thread/lock_guard.html"><span class="kw2170">std::<span class="me2">lock_guard</span></span></a><span class="sy1">&lt;</span><a href="../thread/mutex.html"><span class="kw2158">std::<span class="me2">mutex</span></span></a><span class="sy1">&gt;</span> lock<span class="br0">(</span>cout_mutex<span class="br0">)</span><span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"Rage counter for main: "</span> <span class="sy1">&lt;&lt;</span> rage <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    a.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    b.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Rage counter for a: 2
Rage counter for main: 1
Rage counter for b: 2</pre></div></div> 
</div>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/216.html">CWG 216</a>
</td>
<td> C++98
</td>
<td> unnamed class and enumeration in class scope have<br>different linkage from those in namespace scope
</td>
<td> they all have external<br>linkage in these scopes
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/389.html">CWG 389</a>
</td>
<td> C++98
</td>
<td> a name with no linkage should not be<br>used to declare an entity with linkage
</td>
<td> a type without linkage shall not be used<br>as the type of a variable or function<br>with linkage, unless the variable<br>or function has C language linkage
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/426.html">CWG 426</a>
</td>
<td> C++98
</td>
<td> an entity could be declared with both internal<br>and external linkage in the same translation unit
</td>
<td> the program is ill-formed in this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/527.html">CWG 527</a>
</td>
<td> C++98
</td>
<td> the type restriction introduced by the resolution of CWG<br>389 was also applied to variables and functions that<br>cannot be named outside their own translation units
</td>
<td> the restriction is lifted for these<br>variables and functions (i.e. with no<br>linkage or internal linkage, or declared<br>within unnamed namesapces)
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/809.html">CWG 809</a>
</td>
<td> C++98
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">register</span></span></span> served very little function
</td>
<td> deprecated
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1648.html">CWG 1648</a>
</td>
<td> C++11
</td>
<td> <code>static</code> was implied even if<br><code>thread_local</code> is combined with <code>extern</code>
</td>
<td> implied only if no other storage<br>class specifier is present
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1686.html">CWG 1686</a>
</td>
<td> C++98<br>C++11
</td>
<td> the name of a non-static variable declared in namespace<br>scope had internal linkage only if it is explicitly<br>declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> (C++98) or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> (C++11)
</td>
<td> only required the type<br>to be const-qualified
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2019.html">CWG 2019</a>
</td>
<td> C++98
</td>
<td> the storage duration of reference members were unspecified
</td>
<td> same as their complete object
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2387.html">CWG 2387</a>
</td>
<td> C++14
</td>
<td> unclear whether const-qualified variable<br>template have internal linkage by default
</td>
<td> const qualifier does not affect<br>the linkage of variable<br>templates or their instances
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

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